In 1977-78, we have continued testing the hypothesis that treatment with 1-isoproterenol may enhance toxicity by decreasing the "average total body clearance" of many compounds, including drugs that are not metabolized significantly and are excreted rapidly and almost entirely by the kidneys. L-Isoproterenol (0.3 mg base/kg s.c.), administered within 10 sec of the i.v. injection of a test compound, decreased the "average total body clearance" of gentamicin, furosemide, and a small dose (0.1 mg/kg) but not a large dose (1.0 mg/kg) of digoxin. The LD50 value of furosemide was reduced by isoproterenol. In order to better fit the requirements for pharmacokinetic analysis of data, we have begun using "mini-pumps" for constant infusions of isoproterenol.